Buildings featuring irregularities in the gravity load carrying frames in low-to-moderate seismicity regions
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2020-10-22.
© 2018 Dr Alireza Mehdipanah
The need of building to have column-free space at certain storeys due to architectural and aesthetical reasons, or a change in the functionality of adjacent storeys and building facility requirements, explain why many buildings feature irregularities in their gravity load carrying frames. However, discontinuities in the load-bearing system may result in unfavourable failure mechanisms under severe earthquake excitations. These buildings commonly exist in low to moderate seismic regions, like Australia; given that strict regulations do not exist with respect to earthquake resistant design of buildings. Furthermore, in these regions buildings typically have non-ductile detailing and are therefore prone to brittle and sudden failures. There is a growing need to assess the performance of vulnerable buildings in low to moderate seismic regions as it has been acknowledged that these regions are exposed to the risk of rare occurrence of devastating and damaging earthquakes. This research aims at investigating the seismic performance of a class of vertically irregular buildings in regions of low to moderate seismicity in order to provide a broad and comprehensive understanding of their response behaviour. Owing to the importance of vulnerability assessment of buildings featuring the use of transfer beams in the gravity load carrying frames (discontinuity or off-set in the load path), linear and nonlinear response behaviour of this class of buildings have been investigated thoroughly. Although “gravity frames” are assumed only to carry vertical loads, the effects of their lateral strength and stiffness need be taken into consideration in the seismic analysis. Results of studies revealed that the elastic response behaviour of vertically irregular buildings is consistent with regular buildings in terms of stiffness, modal periods, modal shapes, and lateral displacement and shear force profiles. Hence, stiffness irregularity (as a consequence of discontinuity) may not develop in the elastic range. An analysis method is known as the Generalised Force Method (GFM) which has been developed to remedy the shortcomings of the Equivalent Static Analysis method is introduced. This method is not subject to height range restrictions and is applicable to buildings that may have eccentricity, and/or transfer beam irregularity. A technique for the modelling of limited ductile beam/column components based on the concentrated plasticity modelling method has been proposed and used to develop the nonlinear models. Damage mechanism, failure patterns and weak regions of these buildings have been investigated using nonlinear analysis methods, and finally, response modification factors have been calculated for these buildings. It has been shown that a weak storey in the cases where the contributions from moment-resisting frames are high can be developed as a consequence of shear failure in the transfer beam. The behaviour of limited ductile shear walls or non-ductile columns governs the seismic behaviour of these buildings. Failure of walls due to the lack of boundary elements may occur as a result of poor detailing. However, shear failure in the transfer beams may also occur prior to the failure of walls in some cases. Hence, to avoid undesirable seismic performance behaviour such as weak storey collapse mechanism (due to the shear failure of transfer beams), more complicated methods of analysis might not necessarily result in a more desirable outcome. Ductility factors for the buildings investigated have been found to be less than 1.5. Hence, current detailing practice may not ensure a ductility value of 2.0 in compliance with AS117.4:2007, even for the regular buildings. The ductility factor for certain irregular buildings which typically has higher contributions from moment-resisting frames to the lateral stiffness have been found to be close to 1.0; which reveals the concentration of plasticity in the critical elements while elastic response behaviour is experienced with the other elements. Hence, the structure is almost entirely elastic at the onset of developing a mechanism. Guidelines and recommendations for the design of a new building in the form of designation of applicable analysis methods for these buildings and response modification factors are provided in the thesis. A straightforward method for the seismic assessment of limited ductile shear wall dominant buildings has been recommended. The method uses a rational approach to predict the nonlinear response behaviour of a given building by modifying the stiffness of individual members of an elastic model for the building structure. Moreover, a probabilistic rapid assessment tool for generating fragility curves is proposed for these buildings. This tool can be used for obtaining information for the risk assessment studies, by providing a simple and rapid method to assist global decision makers.
Keywordsseismic assessment; seismic design; irregularity; transfer beam; limited ductile; low-to-moderate seismicity
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